1. Field of the Invention
The invention relates to the field of combustion technology. It refers to a pressure atomizer nozzle, comprising a nozzle body with a mixing chamber which is connected to an outside space via a nozzle bore. The nozzle body has a first feed duct for a liquid to be atomized, through which duct said liquid can be fed under pressure, free of swirling, to this chamber. At least one further feed duct for a portion of the liquid to be atomized or for a second liquid to be atomized opens into the chamber of the nozzled body, through which duct said portion of liquid or the second liquid can be fed under pressure and with swirling. A nozzle of this type is known, for example, from DE 196 08 349.4.
2. Discussion of Background
Atomizer burners, in which the oil undergoing combustion is finely distributed mechanically, are known. The oil is decomposed into fine droplets of a diameter of about 10 to 400 .mu.m (oil mist) which, whilst mixing with the combustion air, are evaporating in the flame and are burnt. In pressure atomizers (see Lueger-Lexikon der Technik [Lueger Lexicon of Technology], Deutsche Verlags-Anstalt Stuttgart, 1965, Volume 7, page 600), the oil is fed under high pressure to an atomizer nozzle by means of an oil pump. The oil passes via essentially tangentially extending slits into a swirl chamber and leaves the nozzle via a nozzle bore. This ensures that the oil particles acquire two movement components, an axial and a radial. Due to centrifugal force, the oil film emerging as a rotating hollow cylinder from the nozzle bore widens to form a hollow cone, the edges of which begin to vibrate in an unstable manner and break up into small oil droplets. The atomized oil forms a cone having a greater or lesser aperture angle.
However, in the low-pollutant combustion of mineral fuels in modern burners, for example in premixing burners of the double cone type, the basic design of which is described in EP 0 321 809 B1, special requirements are placed on the atomization of the liquid fuel. These are primarily as follows:
1. The droplet size must be small, so that the oil droplets can evaporate completely prior to combustion.
2. The aperture angle (angle of spread) of the oil mist should be small, particularly in the case of combustion under increased pressure.
3. The drops must have high velocity and high momentum, so as to be capable of penetrating sufficiently far into the compressed mass stream of combustion air, so that the fuel vapor can be premixed completely with the combustion air before it reaches the flame front.
Swirl nozzles (pressure atomizers) and air-assisted atomizers of known types, with a pressure of up to about 100 bar, are scarcely suitable for this purpose, since they do not allow a small angle of spread, the atomization quality is restricted and the momentum of the drop spray is low.
In the case of swirl-stabilized burners (for example burners of the double cone type), in which flame stabilization is achieved with the aid of a swirl flow, the region between the swirl generator and the recirculation zone, which occurs due to the swirl flow bursting open, is suitable for mixing in and evaporating the liquid fuel. To achieve good preevaporation, the fuel should be introduced, finely atomized, into the flow, which can be carried out in the simplest way by means of a pressure atomizer nozzle. If the fine droplets are exposed to a swirl flow field, however, this may cause the drops to be thrown out because of the centrifugal forces (cyclone effect). The result of wetting the swirl generator or the mixing tube walls would be that mixing would be impaired and there would be the risk of flashback along the walls and deposits occurring due to fuel decomposition.
As a consequence of this insufficient evaporation and premixing of the fuel, therefore, it is necessary for water to be added in order to lower the flame temperature and consequently prevent NOx formation locally. Since the water supplied also often disturbs flame zones which, although per se generating only a small amount of NOx, are very important for flame stability, instabilities, such as flame pulsation and/or poor burnout, frequently occur, thus leading to an increase in CO emission.
An improvement can be achieved by means of the high pressure atomization nozzle known from EP 0 496 016 B1. This consists of a nozzle body, in which a turbulence chamber is designed, said turbulence chamber being connected to an outside space via at least one nozzle bore and having at least one feed duct for the liquid to be atomized which is capable of being fed under pressure. Said nozzle is defined in that the cross-sectional area of the feed duct opening into the turbulence chamber is larger by the factor 2 to 10 than the cross-sectional area of the nozzle bore. This arrangement makes it possible, in the turbulence chamber, to generate a high turbulence level which does not die out on the way to the outlet of the nozzle. The liquid jet is rapidly decomposed by the turbulence generated in front of the nozzle bore in the outside space, that is to say after leaving the nozzle bore, low angles of spread of 20 .degree. and less being obtained. The droplet size is likewise very small.
When gas turbine burners are being operated with liquid fuel, the aim is to generate a drop spray, if possible over the entire load range of the gas turbine (approximately 10% to 120% fuel mass flow in relation to rated load conditions), said spray making it possible in the entire range to achieve low-pollutant and stable combustion in a predetermined air flow field.
The use of an above described high pressure atomizer nozzle for the atomization of liquid fuel in gas turbine burners certainly leads, as desired, under full load and overload (100-120%) to a pressure (100 bar) which is not too high and to a small droplet size, undesirable wall wetting and coking being avoided on account of the narrow spray angle.
Under part load, however, the fuel admission pressure drops because of the falling overall fuel mass flow. Yet the energy for pressure atomizers, which is necessary for atomization, is determined by the fuel admission pressure, so that, in this load range, the atomization quality is impaired and the depth of penetration of the fuel spray into the air flow decreases due to the low fuel admission pressure.
This disadvantage is overcome by means of the two-stage pressure atomizer nozzle according to DE 196 08 349.4 which has already been mentioned. This is operated via a swirlfree main turbulence generating stage in the full load and overload mode and additionally or else solely via a pressure swirl stage in the part load and low load mode. The disadvantage of this solution, however, is that, because of the high turbulence in the jet of the main turbulence generating stage, it is not possible to have very narrow spray angles (&lt;15.degree.). For specific instances of use, in which the burner air is sharply swirled, however, very narrow fuel jet angles are necessary in order to avoid the walls being coated. In principle, jet nozzles, so-called plain jets, are suitable for this purpose. These, however, produce atomization which is somewhat unsuitable for igniting the burner.